A conductive rubber material composed of a rubber material and an electrically conductive material is used as a flexible conductive material for various electric and electronic parts. Especially, a conductive elastomer film is used for a conductive collector, an antistatic material, an electromagnetic wave-shielding material, an electrode, a connector, a sensor and a heating element, and a conductive elastomer film having a more reduced electric resistance is desired for these applications.
Electronic circuits occasionally make a wrong operation because noises are generated from irradiation with electromagnetic waves produced by the circuits themselves or by external factors. Therefore, in order to minimize the influence of electromagnetic waves, in most electronic circuits, each unit circuit or the whole apparatus having the electronic circuits is covered by a conductive electromagnetic wave-shielding material. As the electromagnetic wave-shielding material, metal cases or metal covers are generally used. However, the metal cases or metal covers have a large size and a special shape such that they can be provided so as not to be in contact with circuit parts, and large spacings are kept between the circuit parts. Especially, in portable electronic instruments, which are required to be small in size and lightweight, electronic circuits are packaged therein in close vicinity to each other, and thus, thin and pliable electrically-conductive film materials are desired as substitutes for the metal shielding material.
An electric double-layer capacitor is an electric element comprising a polarizable electrode and an electrolyte, and, as the electrolyte, an electrolytic solution of an electrolyte salt is generally used [Japanese Unexamined Patent Publication (hereinafter abbreviated to "JP-A") S49-68254 and others]. The electric double-layer capacitor has a function of storing an electric charge in an electric double-layer formed between the electrode and an electrolyte by the polarization of the electrode. The electric double-layer capacitor is used as a small-size electric source for back-up semiconductor memory and others, which are required to be smaller in size and more high-performance. The electric double-layer capacitor is expected to be used as an auxiliary power source for an electric vehicle and a fuel cell vehicle.
The double-layer capacitors heretofore used are classified into two types: the first type using an aqueous electrolyte, usually aqueous sulfuric acid solution having a concentration of about 25 to 50% by weight (for example, JP-A S62-268119, JP-A S63-213915 and JP-A H2-174210) and the second type using an organic solvent electrolyte (JP-A S49-68254 and JP-A H7-86096). In general, an organic solvent electrolyte has a high output voltage, but it has a low Ionic conductivity, and thus, results in increase of internal resistance and reduction of output current. In contrast, an aqueous electrolyte has a low voltage, but it has a high ionic conductivity, and thus, results in reduction of internal resistance and increase of output current.
In addition, an electric double-layer capacitor using an organic solvent electrolyte is combustible, and thus, an electric double-layer capacitor using an aqueous electrolyte and having a high output is desired from a safety viewpoint.
In a large electric source having an electric double-layer capacitor using an aqueous electrolyte and a plurality of cells arranged in series or parallel, if it is intended to enhance the output power, the internal resistance must be reduced to increase the output current. The internal resistance occurs due to the aqueous electrolyte, carbon of the electrode, conductive collector and other factors. The internal resistance occurring due to the aqueous electrolyte and carbon of the electrode can be lowered by reducing the thickness of cells, but the thinning of cells leads to reduction of cell capacity. Therefore it is very effective to reduce the volume resistance of the conductive collector.
As the collector, rubber films containing an electrically conductive material are generally used (for example, JP-A H2-174210, JP-A H4-240708 and JP-A H5-299296). However, the conventional conductive rubber films have a volume resistance of about 8 to 500 .OMEGA.cm as measured in the direction perpendicular to the film plane, and thus, it is difficult to enhance the output power of the capacitor. In addition, the conventional conductive rubber films have a poor acid resistance and, when the capacitor is used for a long period, the rubber films are eroded, and their function is reduced and leakage of electrolyte is liable to occur.
A conductive sheet having a conductive rubber layer having a volume resistance not larger than 10 .OMEGA.cm and comprising a polymer and carbon black is described in JP-A H8-106816. As specific examples of the polymer, there can be mentioned butyl rubber, halogenated butyl rubber and ethylene-propylene copolymer rubber. These polymers generally have a problem such that, when a large amount of carbon black is incorporated, they exhibit a poor solubility in a solvent, and, a conductive film having a high uniformity is difficult to produce from the polymer solution. Thus, a conductive film, which is homogeneous and has a low volume resistance as measured in the direction perpendicular to the film plane, is difficult to produce. Further, a conductive film having good mechanical strengths and good resistance to gas permeability also is difficult to produce from these polymer.